1. Field of the Invention
The present invention relates generally to a wireless communication system, and in particular, to a method and apparatus for transmitting and receiving control channels in an Orthogonal Frequency Division Multiple Access (OFDMA) system.
2. Description of the Related Art
Recently, in wireless communication systems, intensive research is being conducted on Orthogonal Frequency Division Multiplexing (OFDM) and Orthogonal Frequency Division Multiple Access (OFDMA) as a useful scheme for high-speed data transmission in wireless channels.
OFDM, a scheme for transmitting data using multiple carriers, is a type of Multi-Carrier Modulation (MCM) that converts a serial input symbol stream into parallel symbol streams and modulates each of the parallel symbol streams with multiple orthogonal subcarriers or subcarrier channels before transmission.
FIG. 1 is a diagram illustrating a structure of a transmitter in a conventional OFDM system. Referring to FIG. 1, an OFDM transmitter includes an encoder 101, a modulator 102, a serial-to-parallel converter 103, an Inverse Fast Fourier Transform (IFFT) block 104, a parallel-to-serial converter 105, and a Cyclic Prefix (CP) inserter 106. The encoder 101, i.e., a channel encoding block, performs channel encoding on a specific input information bit stream. Generally, a convolutional encoder, a turbo encoder, or a Low Density Parity Check (LDPC) encoder is used as the encoder 101. The modulator 102 generates modulation symbols by performing modulation, such as Quadrature Phase Shift Keying (QPSK), 8-ary Phase Shift Keying (8PSK), 16-ary Quadrature Amplitude Modulation (16QAM), 64QAM, and 256QAM, on the output of the encoder 101. Although not illustrated in FIG. 1, a rate matching block for performing repetition and puncturing can be further included between the encoder 101 and the modulator 102. The serial-to-parallel converter 103 serves to convert the serial output of the modulator 102 into parallel data.
The IFFT block 104 performs an IFFT operation on the output of the serial-to-parallel converter 103. The output of the IFFT block 104 is converted into serial data by the parallel-to-serial converter 105. Thereafter, the CP inserter 106 inserts a CP code into the output of the parallel-to-serial converter 105.
The Long Term Evolution (LTE) system, which is now under discussion as the next generation wireless communication system of the Universal Mobile Telecommunication Service (UMTS) system in the 3rd Generation Partnership Project (3GPP) standardization organization, uses Single Carrier Frequency Division Multiple Access (SC-FDMA) for the uplink to solve the high Peak-to-Average Power Ratio (PAPR) problem of OFDMA. SC-FDMA, a type of OFDM, can be realized by adding an FFT block to a front of the IFFT block 104 and pre-coding pre-IFFT data.
FIG. 2 conceptually illustrates resources of a conventional OFDM system. As illustrated in FIG. 2, in OFDM or SC-FDMA, wireless resources are expressed as a two-dimensional time-frequency array. More specifically, the horizontal axis represents a time domain 201 and the vertical axis represents a frequency domain 202. In the time domain 201, 7 OFDM symbols constitute one slot 204, and two slots constitute one subframe 205. Generally, one subframe 205 has the same length as a Transmission Time Interval (TTI), which is the basic transmission unit.
FIG. 3 illustrates a data transmission and reception procedure between a base station and a terminal in a conventional OFDM system. Referring to FIG. 3, in step 303, a terminal 302 generates a Channel Quality Indicator (CQI) indicating a downlink channel state based on a received Reference Signal (RS) transmitted by a base station 301, and transmits the CQI to the base station 301 in step 304. In this case, the terminal 302 can transmit a Channel Sounding Reference Signal (CS/RS) along with the CQI in order for the base station 301 to identify the uplink channel state.
Upon receiving the CQI and/or the CS/RS, the base station 301 determines downlink or uplink resources it will allocate to the terminal 302 through scheduling in step 305, and transmits a scheduling grant indicating the determined downlink/uplink resources to the terminal 302 in step 306. The terminal 302 first determines if the scheduling grant has been delivered to the terminal 302 itself. If the scheduling grant has been delivered to the terminal 302, the terminal 302 recognizes the allocated downlink/uplink resources indicated by the scheduling grant in step 307, and exchanges data with the base station 301 using the allocated downlink/uplink resources in step 308.
The base station 301 delivers the information necessary for data transmission/reception to the terminal 302 using a scheduling grant, and the scheduling grant is transmitted to the terminal 302 through a Physical Downlink Control Channel (PDCCH). The PDCCH uses some of the resources illustrated in FIG. 2. The base station 301 selects one or multiple PDCCHs from among a plurality of available PDCCHs, and transmits the scheduling grant to the terminal 302 through the selected PDCCH(s).
Because the terminal 302 does not know which channel among the multiple PDCCHs transmitted by the base station 301 is used for the terminal 302, the terminal 302 must monitor all PDCCHs transmitted by the base station 301 to determine if there is a PDCCH having a scheduling grant being transmitted to the terminal 302. In this case, if the number of PDCCHs transmitted by the base station 301 is great, the terminal 302 must perform a large number of reception operations to check the scheduling grant, requiring complicated reception structure and increasing power waste of the terminal.